1. Field of the Invention
The present invention relates, in general, to the technology of temperature measurement and, more particularly, to temperature measurement through the use of a thermocouple. In further detail, the present invention relates to a thermocouple assembly which is designed to protectively retain a thermocouple in a high pressure environment in a manner so as not to impair the response time of the thermocouple.
2. Discussion of the Prior Art
In the manufacture of polyethylene, the accurate control of the temperature present within a reactor is essential in order to ensure the production of high quality polyethylene product. Early polyethylene reactors generally utilized a thermocouple installed in a thermowell wherein a heat conductive material, such as an oil or silicone, was placed in the thermowell to ensure a good thermal connection or high degree of heat transfer between the well and the thermocouple. Subsequent to the use of this arrangement for several years, a thermocouple was designed which would satisfactorily withstand the great pressures reigning within the polyethylene reactor and, accordingly, would be capable of being directly immersed in the reactant materials being processed in the reactor without necessitating the use of a protective thermowell. While the use of a thermocouple which could be directly installed in the reactor process medium represented an improvement over the previous type of thermocouple installed in a thermowell, the thermocouples which were utilized could not detect temperature changes rapidly enough to enable the taking of measures which would prevent deleterious run-away reactions from being generated within the reactor or to result in the production of materials not meeting their manufacturing specifications.
The response time of these prior art thermocouples fairly well matched the response time of the control equipment for the polyethylene reactor which was available in the technology at that time. However, in recent years control equipment has been developed which has much faster response times. In view of the rapid response time of present control equipment, the sometime slower response time of the thermocouple has become a limiting factor in the speed of response for the overall reactor control system.
Generally, the response time of a thermocouple is defined as the elapsed time which is required for the thermocouple junction to reach 63.2% of its final temperature when exposed to, successively, two different temperature levels. The response time is typically influenced by several factors, including, the type and size of wire utilized to form the elements of the thermocouple junction, the size of the thermocouple junction weld, and the heat sink effect of protective sheaths utilized to hold and encompass the thermocouple. Heretofore, it has been the practice in the art to install thermocouples within a relatively heavy stainless steel sheath in order to ensure that the thermocouple assembly has adequate mechanical strength for use in polyethylene reactors. The protective sheath acts as a heat sink, and, accordingly, becomes a very significant factor in degrading the thermocouple response time.
Thus, it becomes desirable to provide a thermocouple assembly which is designed to operate in a high pressure environment of up to 60,000 psi, such as in a polyethylene reactor, which will operate in a high temperature environment of up to 600.degree. F., has adequate mechanical strength allowing it to be utilized in the reactor, and also has a rapid response time, in a magnitude of less than 0.2 seconds, so as to enable it to quickly sense any changes of temperature taking place within the reactor.
Maurer U.S. Pat. No. 3,811,958 discloses a thermocouple assembly in which the sensing thermocouple junction is encased in a metallic cylindrical sheath casing which is joined to a larger cylindrical casing. Maurer is not at all concerned with operation in a high pressure environment, and the joinder of the two cylindrical sections of the thermocouple would not provide an adequate pressure seal for a high pressure environment. Further, this patent does not disclose a duplex thermocouple assembly, but instead has structure providing for only one thermocouple junction.
Sabovik U.S. Pat. No. 3,589,192 discloses a thermocouple assembly in which an elongated sheathed thermocouple is received in a bore in a mounting body member. The sheathed thermocouple is releasably held in the mounting body so as to be movable or displaceable axially relative to the body member, and thereby provide an adjustment whereby the thermocouple junction may project varying distances from the end of the body member. The presence seal in this thermocouple assembly is formed by a compressible gasket arranged between the two movable members and which would afford inadequate sealing in a high pressure environment.
Pustell U.S. Pat. No. 3,539,400 illustrates a thermocouple assembly which is formed of two concentric cylindrical material sheaths in which the outer cylindrical sheath completely encloses the inner cylindrical sheath. The construction of this thermocouple assembly is designed to equalize thermal expansion between the various sheaths of the assembly. Pustell is not at all concerned with adequate sealing of the thermocouple assembly against the adverse effects of operation in a high pressure environment.
McCall U.S. Pat. No. 3,284,247 shows a thermocouple assembly wherein the thermocouple wires are placed in a pair of axial channels which are formed in the thermocouple surface and extend along the axial length of one element of the assembly. However, the thermocouple wires are merely placed within these channels for purposes of guidance, and the thermocouple assembly disclosed in McCall is not designed to withstand inordinately high operating pressures.
Klicks U.S. Pat. No. 3,996,071 discloses a thermocouple assembly designed to measure temperature in extremely high pressure environments, but is not concerned with whether the response time of the thermocouple is impaired by the protective assembly. Accordingly, the design of the Klicks thermocouple assembly is of a completely different nature than that of the design of the present invention.